Why is it that when ink is spilled it dries up, but when it is in the bottle it does not dry up?
What put the salt into the ocean?
Why do you feel cold when you get out of the bathtub?
Wet clothes get dry when they are hung on the clothes-line. The water in them evaporates. It turns to invisible vapor and disappears into the air. Water and all liquids evaporate when they are long exposed to the air. If they didn't—well, let us imagine what the world would be like if all evaporation should suddenly stop:
You find that your face is perspiring and your hands as well. You wipe them on your handkerchief, but soon they are moist again, no matter how cool the weather. After wiping them a few more times your handkerchief becomes soaking wet, and you hang it up to dry. There may be a good breeze stirring, yet your handkerchief does not get dry. By this time the perspiration is running off your face and hands, and your underclothes are getting drenched with perspiration.
Fig. 48. An evaporating dish.
You hurry into the house, change your clothes, bathe and wipe yourself dry with a towel. When you find that your wet things are not drying, and that your dry ones are rapidly becoming moist, you hastily build a fire and hang your clothes beside it. No use, your clothes remain as wet as ever. If you get them very hot the moisture in them will boil and turn to steam, of course, but the steam will all turn back to water as soon as it cools a little and the drops will cling to your clothes and to everything around the room. You will have to get used to living in wet clothes. You won't catch cold, though, since there is no evaporation to use up your heat.
But the water problem outside is not one of mere inconvenience. It never rains. How can it when the water from the oceans cannot evaporate to form clouds? Little by little the rivers begin to run dry—there is no rain to feed them. No fog blows in from the sea; no clouds cool the sun's glare; no dew moistens the grass at night; no frost shows the coming of cold weather; no snow comes to cover the mountains. In time there is no water left in the rivers; every lake with an outlet runs dry. There are no springs, and, after a while, no wells. People have to live on juicy plants. The crops fortunately require very little moisture, since none evaporates from them or from the ground in which they grow. And the people do not need nearly as much water to drink.
Little by little, however, the water all soaks too deep into the ground for the plants to get it. Gradually the continents become great deserts, and all life perishes from the land.
All these things would really happen, and many more changes besides, if water did not evaporate. Yet the evaporation of water is a very simple occurrence. As the molecules of any liquid bounce around, some get hit harder than others. These are shot off from the rest up into the air, and get too far away to be drawn back by the pull of the molecules behind. This shooting away of some of the molecules is evaporation. And since it takes heat to send these molecules flying off, the liquid that is left behind is colder because of the evaporation. That is why you are always cold after you leave the bathtub until you are dry. The water that evaporates from your body uses up a good deal of your heat.
